Positive displacement pumps are used in a number of different industrial and commercial processes to force fluid movement from a first location to a second location. One type of positive displacement pump that is often used when such fluid transport is required is the rotary vane pump. A rotary vane pump includes a housing, a section of which is shaped to define a pump chamber. Often, the pump chamber has an eccentric, non-circular cross-sectional profile. In prior art pumps of this type, flat, stationary discs define the front and rear ends of the chamber. A shaft extends through the housing. Attached to the shaft is a rotor that is inwardly spaced relative to the inner wall of the casing that defines the pump chamber. Vanes extend outwardly from slots in the rotor. As the shaft and rotor turn, the volume of the space in the chamber between adjacent vanes and the opposed surfaces of the rotor and housing, referred to as a fluid cavity, cyclically increases and decreases. As a result of the volume of a fluid cavity increasing, a suction is formed in the cavity. The suction draws fluid into the fluid cavity through an inlet opening. As the rotor continues to turn, owing to the geometry of the pump chamber, the volume of the fluid cavity decreases. As a result of the volume of the cavity decreasing, the fluid in the cavity is discharged through an outlet opening.
At any given moment during the actuation of a rotary vane pump, the section of the rotor adjacent where the fluid is being discharged is subjected to a pressure force. The other sections of the rotor are not subjected to like stress. In other words, during the normal operation of a rotary vane suction pump, the pump rotor and, more significantly, the shaft to which the rotor is attached, is subjected to uneven, asymmetric, loading. It is presently common practice to rotatably suspend the pump shaft in the associated casing with two spaced apart bearing assemblies. The rotor is mounted over the shaft so as to be located between the bearing assemblies. More specifically the portion of the rotor mounted to the shaft is referred to as the hub. The pressure load on the rotor is transmitted through the hub to the shaft and through the opposed ends of the shaft to the bearing assemblies.
As a consequence of the above arrangement, the size of the rotor is, to a significant extent, linked to the size of the shaft to which the rotor is mounted. This relationship can sometimes lead to design disadvantages. For example, in order to minimize the unit area shaft stress, a specific sized shaft is needed in order to provide a pump capable of being exposed to a specific maximum pressure load. An inherent consequence of increasing shaft size, shaft diameter, is that the size, diameter, of the associated rotor also increases. In order to provide the desired internal velocity of the fluid cavities, it is typically necessary to rotate these shaft-rotor assemblies at relatively slow speeds. This typically results in having to provide a speed reducer assembly between the motor used to drive the pump and the associated pump shaft.
Still another consequence of providing a pump of the above design is that it requires the placement of dynamic seals around both ends of the rotor. Providing two of these seals adds to the costs of both constructing and maintaining the pump.